This invention relates to high performance analog-to-digital (A/D) converters and magnetometers and, in particular, to A/D converters and magnetometers employing superconducting quantum interference devices (SQUIDs) and having a wide dynamic range.
SQUIDs include the combination of an inductance and one or more Josephson junctions. Josephson junctions have a current-voltage (I-V) characteristic of the type shown in FIG. 1 and, for ease of illustration, Josephson junctions are shown by an "X", in the appended drawings. Initially, prior to being powered, each Josephson junction is in a "superconductive" state (S-state) and functions as a short circuit, i.e., its resistance [and/or impedance] is zero. A Josephson junction remains in the S-state until the current through the Josephson junction exceeds the critical current (Ic) of the device. When the critical current (Ic) of the Josephson junction is exceeded, the device is then switched to what is termed the "normal" state (N-state). In going from the "S" state to the "normal" state, the characteristic of the device changes abruptly as shown in FIG. 1. In the "S" state, a Josephson junction exhibits zero impedance and zero voltage drop for current through the device below the critical current (Ic) of the device. In the "normal" state which may also be termed the "voltage" or "resistive" state, the Josephson junction exhibits a very high impedance for voltages of less than, for example, 2.5 millivolts and a somewhat lower but still significant impedance for voltages in excess of, for example, 2.5 millivolts applied or developed across the Josephson junction.
A superconducting quantum interference device (SQUID) is a circuit which includes one or more Josephson junctions and one or more inductive loads. A single junction SQUID includes the combination of a single Josephson junction connected across an inductance. A double junction SQUID includes the combination of an inductance and two Josephson junctions as shown, for example, in SQUIDs 82 and 84 of FIG. 2 and SQUIDs 24, 34, S1 and S2 of FIG. 3. A current may be injected into one end of, or across, the inductance of the SQUID and one end of each Josephson junction is connected to the SQUID inductor, and the other end of the Josephson junction may be returned to ground or some point of reference potential as shown in FIGS. 2, and 3.
It is known in the art to use single and double junction SQUIDs in analog,to-digital (A/D) converter and digital magnetometer systems as illustrated in U.S. Pat. No. 4,672,359 titled SUPERCONDUCTING ANALOG-TO-DIGITAL CONVERTER AND DIGITAL MAGNETOMETER AND RELATED METHOD FOR ITS USE issued to Arnold H. Silver; the teachings of which are incorporated herein by reference. It is also known in the art to operate a SQUID circuit as a "digital SQUID". In a particular application the "digital SQUID" is a SQUID which is operated as a comparator such that it receives an unknown magnetic flux and produces a comparator output in the form of a pulse sequence. This is described, for example, in an article titled JOSEPHSON INTEGRATED CIRCUITS III; A SINGLE-CHIP SQUID MAGNETOMETER by NORIO FUJIMAKI published in FUJITSU SCI. TECH. J. 27, 1, pp. 59-83 (April 1991); the teachings of which are also incorporated herein by reference.
The latter reference describes a single chip SQUID Magnetometer reproduced in FIG. 2 of this application. The circuit of FIG. 2 has many desirable features. However, it suffers from low dynamic range as explained below. In FIG. 2, the input signal source includes a first loop comprising a pick up coil 81 and an inductor L81. Signals in the first loop are coupled to a first SQUID sensor circuit which includes inductor L81 and a double junction SQUID 82 comprised of inductor L82 and Josephson junctions J82A and J82B. An AC bias source 83 is coupled to an input/output node 84. A resistor R81 and an inductor L83 are connected in series between node 84 and a node 85. A second SQUID circuit 84 comprised of inductor L84 and two Josephson junctions J84A, J84B is connected to node 85. Inductor L83 and SQUID 84 define what is referred to herein as a write gate. Two inductors, Lf1 and Lf2, are connected in series between node 85 and ground.
In the circuit of FIG. 2, AC BIAS source 83 produces bipolarity pulses which are applied to node 84 and SQUID 82.
In response to the presence of an analog input signal in coil L81 which is greater than zero and in the presence of bipolar pulses from bias source 83, the comparator SQUID produces either positive going pulses for one polarity of input signals, or negative going pulses for input signals of opposite polarity, to the one polarity. When the input signal is zero the comparator produces alternately positive going or negative going pulses. The pulses produced by the comparator SQUID 82 are supplied to a feedback circuit which includes a Write gate comprised of L83 and SQUID 84 comprised of L84, J84A and J84B. The write gate produces a feedback signal which increases (positively or negatively) by a predetermined value for each pulse (positively or negatively) produced by the comparator SQUID. The increasing feedback signal is fed back via inductors Lf1 and Lf2 to the SQUID sensor. The feedback signal increases until the amplitude of the magnetic flux fed back into L82 is equal and opposite to the input flux in L81, so as to cancel the effect of the input flux. The build up of the flux fed back in and to the SQUID sensor limits the dynamic range of the circuit. This is undesirable because it limits the maximum amplitude signals that can be measured.
The problem of limited dynamic range is significantly reduced in circuits embodying the invention.